1. Field of the Invention
This invention relates generally to a disc player for reproducing optically an information signal from an optical disc, such as an optical digital audio disc, on which the information signal is recorded in a record track formed with an arrangement of a plurality of small pits, and more particularly, is directed to an optical disc player provided with an improved device for generating a tracking error signal and a focus error signal.
2. Description of the Prior Art
In an optical disc player for reproducing an information signal from an optical disc, such as an optical digital audio disc, on which the information signal is recorded in the form of small pits arranged in a spiral track, a light beam is used for reading the information signal from the spiral track on the optical disc. Such a light beam is emitted from an optical head which is moved in the direction of the radius of the optical disc, and required to trace correctly the spiral track and to be focused correctly on a surface of the optical disc on which the spiral track is formed. For making the light beam comply with such requirements, tracking control and focus control are performed. In the tracking control, a position of a beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track thereon is detected to produce a tracking detection output and a focusing lens in the optical head or the optical head in its entirety is moved in the direction of the radius of the optical disc in response to the tracking detection output, and in the focus control, a focusing condition of the light beam on the surface of the optical disc is detected to produce a focus detection output and the focusing lens in the optical head or the optical head in its entirety is moved in the direction perpendicular to the surface of the optical disc in response to the focus detection output.
With regard to detection of the position of the beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track, there have been proposed several detecting arrangements classified into two types, one of which uses two specific light beams provided in addition to the light beam for reading the information signal and the other of which does not use any additional light beam other than the light beam for reading the information signal. As arrangements of the type operative without two specific light beams, such detecting arrangements as called "push-pull system" and "heterodyne system" have been known. In these detecting arrangements, a reflected light beam from the optical disc, which is modulated in intensity at the surface of the optical disc, is projected onto a photodetector comprising a plurality of photodetecting elements to form a beam spot with its pattern varying in response to the position of the beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track, and variations in the pattern of the beam spot on the photodetector are derived in the form of an electric signal obtained through calculating operation among the outputs of the photodetecting elements constituting the photodetector.
On the other hand, with regard to detection of the focus condition of the light beam on the surface of the optical disc, such detecting arrangements as called "astigmatism system" and "gable roof prism system" have been known. In these detecting arrangements also, a reflected light beam from the optical disc, which is modulated at the surface of the optical disc, is projected onto a photodetector comprising a plurality of photodetecting elements to form a beam spot with its pattern varying in response to the focusing condition of the light beam on the surface of the optical disc, and variations in the pattern of the beam spot on the photodetector are derived in the form of an electric signal obtained through calculating operation among the outputs of the photodetecting elements constituting the photodetector.
In previously proposed optical disc players, the signal obtained through the calculating operation among the outputs of the photodetecting elements constituting the photodetector as mentioned above is used without being modified as a tracking error signal representing deviations of the beam spot formed by the light beam on the surface of the optical disc from a center in width of the spiral track or a focusing error signal representing defocus of the light beam on the surface of the optical disc, and such a tracking error signal or focus error signal is limited to a predetermined frequency range by passing through a certain filter circuit, power amplifier circuit and so on to be a tracking control signal which is supplied to a driving device for tracking control to move the focusing lens in the optical head or the optical head in its entirety in the direction of the radius of the optical disc or a focus control signal which is supplied to a driving device for focus control to move the focus lens in the optical head or the optical head in its entirety in the direction perpendicular to the surface of the optical disc. This results in a problem that it is hard to maintain the light beam in correct tracking relation to each turn of the spiral track though the tracking control is conducted or it is hard to maintain correct focus of the light beam incident upon the surface of the optical disc though the focus control is conducted, and further in another problam that a mechanical noise arises from the driving device for tracking control or focus control. Besides, in the case where the optical disc has a wound or stain on the surface thereof, there is a disadvantage that a so-called track jump movement by which the beam spot formed by the light beam on the surface of the optical disc is moved from a certain turn of the spiral track into another turn of the spiral track adjacent thereto is easy to be caused at a portion of the wound or stain.
In the case of the tracking control arrangement, the above mentioned tracking error signal obtained through the calculating operation among the outputs of the photodetecting elements constituting the photodetector takes a level corresponding to the position of the beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track when the beam spot on the surface of the optical disc is positioned on the pit, but takes a constant level irrelative to the position of the beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track when the beam spot on the surface of the optical disc is positioned on a plane portion between two successive pits. Therefore, such a tracking error signal as a whole contains a frequency component resulting from the arrangement of the pits and does not represent exactly the position of the beam spot formed by the light beam on the surface of the optical disc in relation to the spiral track. Consequently, with the tracking control in the previously proposed optical disc player in which the signal obtained through the calculating operation among the outputs of the photodetecting elements constituting the photodetector is used without being modified as the tracking error signal, such a control as to move the light beam to trace exactly the spiral track can not be achieved.
Then, in the case of the focus control arrangement, the pattern of the beam spot on the photodetector varies faithfully in response to the focusing condition of the light beam on the surface of the optical disc regardless of the position of the beam spot on the surface of the optical disc in relation to the spiral track when the beam spot on the surface of the optical disc is positioned on a plane portion between the pits, but varies in response to both the focusing condition of the light beam on the surface of the optical disc and the position of the beam spot on the surface of the optical disc in relation to the spiral track when the beam spot on the surface of the optical disc is positioned fully or partially on the pit. Accordingly, the above mentioned focus error signal obtained through the calculating operation among the outputs of the photodetecting elements constituting the photodetector takes a level corresponding to defocus of the light beam on the surface of the optical disc when the beam spot on the surface of the optical disc is positioned on the plane portion between the pits, but takes a level which varies in response to variations in the position of the beam spot on the surface of the optical disc in relation to the spiral disc and does not correspond to defocus of the light beam on the surface of the optical disc when the beam spot on the surface of the optical disc is positioned fully or partially on the pit. As a result of this, such a focus error signal as a whole not only contains a frequency component resulting from the arrangement of the pits but also is influenced with variations in the position of the beam spot formed by the light beam on the surface of the optical disc and does not represent exactly defocus of the light beam on the surface of the optical disc. Consequently, with the focus control in the previously proposed optical disc player in which the signal obtained through the calculating operation among the outputs of the photodetecting elements constituting the photodetector is used without being modified as the focus error signal, such a control as to cause the light beam to focus exactly on the surface of the optical disc can not be achieved.
In addition to the above, since the tracking error signal or the focus error signal contains the frequency component resulting from the arrangement of the pits, the mechanical noise resulting from such a frequency component arises from the driving device for moving the focusing lens in the optical head or the optical head in its entirety in the direction of the radius of the optical disc or in the direction perpendiculer to the surface of the optical disc when the tracking or focus control signal obtained on the basis of the tracking or focus error signal is supplied to the driveing device.
Further, in the case where the optical disc has a nonreflecting portion with a wound or stain on the surface thereof, a reproduced information signal obtained from the photodetector onto which the reflected light beam from the optical disc is projected is reduced in its level to be extremely low and the tracking error signal becomes zero regardless of the position of the beam spot on the surface of the optical disc in relation to the spiral track when the light beam scans the nonreflacting portion of the surface of the optical disc. Accordingly, the beam spot formed by the light beam on the surface of the optical disc is moved toward a location determined by the neutral position of the focusing lens in the optical head at the nonreflecting portion of the surface of the optical disc, so that the track jump movement of the light beam is easy to be caused. Similarly, when the light beam scans the nonreflecting portion of the surface of the optical disc, the focus error signal is not obtained and therefore a focusing condition of the light beam on the surface of the optical disc determined in response to the neutral position of the focusing lens in the optical head is taken, so that a large amount of defocus of the light beam on the surface of the optical disc is easy to be brought on.